Lithium secondary batteries have progressed as electric power supplies for cellular phones and laptop computers because the energy densities of lithium secondary batteries are high. However, with the reduction in size and weight of portable terminal devices caused by the recent progress of IT technology, demands for the further downsizing and capacity increasing of the batteries as the electric power supplies of such devices have been growing. Additionally, lithium secondary batteries have come to attract attention as electric power supplies for electric automobiles and hybrid automobiles and as electric power supplies for power storage, making the most of the high energy densities of lithium secondary batteries.
As the anode materials of lithium batteries, carbon-based anodes have hitherto been commonly used, and lithium secondary batteries using carbon-based anodes are characterized in that the voltage and the energy density are high at the time of discharge. However, since the potential of the anode is low, rapid charge causes the deposition of lithium metal and increases the risk of causing internal short-circuit, and there is an inherent risk of ignition caused by such internal short-circuit. Under these circumstances, there have been investigated lithium batteries with high safety and long life that are obtained by using a high-potential anode to reduce heat generation at the time of internal short-circuit and suppressing the decomposition of electrolyte solution. Especially Li4Ti5O12 has a potential of 1.5 V based on lithium, does not change in volume at the time of charge or discharge and is extremely good in cycle characteristics, and hence coin batteries using Li4Ti5O12 have been put into practical use.
However, the theoretical capacity of Li4Ti5O12 is 175 mAh/g, which is disadvantageous in that the electric capacity of Li4Ti5O12 is as small as approximately half the electric capacity of carbon, which is commonly used as an anode material, leading to the small energy densities of lithium secondary batteries using Li4Ti5O12. From the viewpoint of their safety and long life, an anode material having a voltage of 1.0 to 1.5 V based on lithium and a large electric capacity has been demanded.
Under such circumstances, titanium-niobium composite oxides have been attracting attention as electrode materials having a voltage of 1.0 to 2.0 V based on lithium and a large electric capacity.
When lithium secondary batteries are used as anode materials, energy density can be increased because titanium-niobium composite oxides can maintain electroneutrality of crystals against intercalation and deintercalation of lithium ions, through a redox reaction between Ti4+ and Nb5+.
As examples of application of titanium-niobium composite oxides, TiNb2O7, Ti2Nb10O29 and TiNb24O62 are under study using a solid-phase method in which TiO2 and Nb2O5 are mixed and fired, and titanium-niobium composite oxides having a specific surface area of 0.18 m2/g or more have exhibited a large electric capacity of 228 to 277 mAh/g (Patent Document 1: Japanese Patent Publication No. 2010-287496). Further, it has been found that C—TiNb2O7 or C—Ti1−yNbyNb2O7, formed by carbon coating on TiNb2O7 or Ti1−yNbyNb2O7 obtained by firing after homogenous mixing of Ti and Nb by a sol-gel method, for the purpose of increasing conductive property and stabilizing the valence state of Nb(IV), have exhibited a large electric capacity of 285 mAh/g in charge or discharge at 1.0 to 2.5 V (Non-patent Document 1: Jian-Tao, Yun-Hui Huang, and J. B. Goodenough, Chemistry of Materials, 23 (2011) 2027-2029. Patent Document 2: Japanese Patent Publication No. 2013-535787). Lithium ion secondary batteries using as an anode material a monoclinic complex oxide comprising TiNb2O7 in which crystallites have been grown in a [001] direction in a solid-phase method have an initial discharge capacity of 261 to 279 mAh/g (Patent Document 3: Japanese Patent No. 5230713).
However, a problem of the titanium-niobium composite oxides obtained by solid-phase methods is that when the size of their particles is reduced by pulverization, the charge and discharge capacity is increased while the cycle characteristics are reduced. It was presumed that a cause of this problem is that the pulverization results in collapse of a part of the crystalline structure, destabilizing the valence state of Nb. In sol-gel methods, low firing temperatures can be set and fine particles of titanium-niobium composite oxides are produced, but their crystallinity is low and hence, the cycle characteristics are poor. For this reason, the cycle characteristics have been enhanced by coating with carbon and the increase of the amount of conducting agent used in preparation of electrodes, but sufficient effects have not been necessarily obtained. Another disadvantage of sol-gel methods is that production cost is high because expensive starting materials are used.